1. Field of the Invention
This invention is concerned with a process for converting synthesis gas, i.e., mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures and oxygenates. In one aspect, this invention is concerned with a process to convert such synthesis gas to hydrocarbon mixtures under conditions permitting good temperature control of the known exothermic reduction of carbon monoxide with hydrogen. In still another aspect, this invention concerns improvements in the process for effecting the hydrogenation of carbon monoxide in the presence of a liquid suspension of catalyst and, in particular, relates to the magnetic separation of catalyst fines and the removal of other undesirable compounds from the wax product which is produced before wax upgrading.
2. Description of the Prior Art
Processes for the conversion of coal and other hydrocarbons, such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels is given in Encyclopedia of Chemical Technology, edited by Kirk-Othmer, Second Edition, Vol. 10, pp. 353-433 (1966), Intersciences Publishers, New York, N.Y., the contents of which are herein incorporated by reference. The particular techniques for gasification of coal or other solid, liquid, or gaseous fuel are not considered to be a part of this invention, although such techniques may be an important consideration in overall process efficiency.
It is considered to be desirable to effectively and more efficiently convert synthesis gas, and thereby coal and natural gas, to highly valued hydrocarbons, such as motor gasoline with high octane number, petrochemical feedstocks, liquefiable petroleum fuel gas, and aromatic hydrocarbons. It is well known that synthesis gas will undergo conversion to form reduction products of carbon monoxide, such as hydrocarbons, at temperatures in the range of from about 350.degree. F. to about 850.degree. F. and under pressures in the range of from about 1 to 1000 atmospheres, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a wide range of products including waxy materials, oxygenates and liquid hydrocarbons, a portion of which have been successfully used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides of iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium with and without promoters.
The range of catalysts and catalyst modifications disclosed in the art encompasses an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen and provides considerable flexibility toward obtaining selected boiling range products. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, edited by Kirk-Othmer, Second Edition, Vol. 4, pp. 446-448, Interscience Publishers, New York, N.Y., the text of which is incorporated herein by reference. See also H. H. Storch, N. Golumbic and R. B. Anderson, "The Fischer-Tropsch and Related Synthesis", John Wiley & Sons, Inc., New York, N.Y.
The hydrogenation of carbon oxides to highly valued hydrocarbons by the Fischer-Tropsch process is highly exothermic and thus, the reaction system must include means to remove the heat of reaction. This is particularly important if a low H.sub.2 /CO ratio synthesis gas is being converted. While low H.sub.2 /CO ratio gas can be produced more cheaply than high H.sub.2 /CO ratio gas, low H.sub.2 /CO ratio gas cannot easily be converted to transportation fuels in conventional, fixed-bed reactors because of the difficulty in temperature control. The high temperatures of reaction and high carbon monoxide partial pressures favor carbon monoxide disproportionation and carbon formation which results in catalyst cementation. This can be shown by the following reaction: EQU 2CO.fwdarw.C+CO.sub.2
For the purpose of furnishing better temperature control for Fischer-Tropsch type synthesis, in particular, if a low H.sub.2 /CO ratio synthesis gas is converted, it has been proposed to suspend the finely divided catalyst in a liquid medium, and preferably a hydrocarbon mixture such as may be, for instance, obtained by way of the higher boiling components of the synthesis products. The suspension can then be subjected to cooling to continuously remove excess heat therefrom.
The slurried-catalyst reactor system otherwise identified as a suspended Fischer-Tropsch catalyst in a liquid medium suitable for the purpose of converting syngas to hydrocarbon products has been the subject of numerous patents. Early patents on the subject are U.S. Pat. Nos. 2,438,029; 2,671,103; 2,680,126; 2,775,607; 2,852,350; and numerous others.
In U.S. Pat. No. 4,252,736, a patent related to the objectives of the present invention as more defined herein below, the conversion of coal to gaseous and liquid products is achieved by the high efficiency gasification of coal to a low H.sub.2 /CO ratio syngas, conversion of the low ratio syngas with a water gas shift slurry Fischer-Tropsch catalyst to a product comprising C.sub.1 to C.sub.80 hydrocarbons and oxygenates, and conversion of the Fischer-Tropsch product to premium gas and increased liquid products comprising gasoline, distillate and lubes over a special zeolite catalyst exemplified by ZSM-5. According to the patent, a coal, coke or coal char gasifier with a low steam to oxygen ratio as well as low steam to coal ratio, such as provided by the British Gas Corporation-Lurgi slagging gasifier, has significant advantages in terms of thermal efficiency and cost and can lead to a reduction of up to 20 to 40% in syngas production costs. This advantage is larger for Eastern bituminous coal which has a lower reactivity. See Table 1 below.
TABLE 1 __________________________________________________________________________ Lurgi Dry Ash Versus BGC-Lurgi Slagger BGC-Lurgi Slagger Frances (Scottish Lurgi Dry Ash Non-Caking North Dakota Coal Western Eastern Reactive) Eastern Lignite __________________________________________________________________________ Gasifier Itself Scf O.sub.2 /mscf Syngas 135 187 168 157 189 lb Steam/mscf Syngas 46 75 10.6 8.6 9.1 Steam/Oxygen 7.5 8.5 1.3 1.15 1.0 H.sub.2 /CO Ratio 2.1 2.8 0.50 0.52 0.5 Cold Gas Efficiency, % 80 76 89 90 91.0 (adjusted for tars) CH.sub.4 /CO + H.sub.2 0.78 0.6 0.32 0.35 0.32 Net Efficiency, % 71 59 78 78 79 Including Shift to Hydrogen-to-CO Ratio to 2 lb Steam/mscf Syngas (Total) 46 75 40 39 39 Net Efficiency, % 71 59 71 70 71 __________________________________________________________________________
However, a penalty is paid for this gain. The product gas of the slagging gasifier has an H.sub.2 /CO ratio of about 0.5 to 0.8 as compared to a ratio of about 2.0 produced by more costly gasifiers. The most advanced known Fischer-Tropsch process, practiced commercially at SASOL in South Africa, requires a synthesis gas with an H.sub.2 /CO ratio exceeding 2:1, produced in a Lurgi Dry Ash gasifier. If synthesis gas were produced in a more economical gasifier in a low H.sub.2 /CO ratio, it would require a shift conversion to increase its H.sub.2 /CO ratio to the level about 2:1 as required. Such shift reaction consumes a considerable amount of energy, especially in the form of steam, largely negating the high thermal efficiency of this gasifier. This is illustrated by the date in Table 1, which lists the steam requirements for shifting a low ratio H.sub.2 /CO gas to a high ratio of 2:1 as well as the reduction in thermal efficiency for the gas production. When using a western type coal (e.g., Frances), shifting to a ratio of 2:1 wipes out the advantage in thermal efficiency. In this operation, a slightly lower steam consumption requirement is offset by the higher oxygen consumption required by the operation. However, when using an eastern coal, the slagging gasifier with a subsequent shift of low ratio H.sub.2 /CO gas to 2:1 is still very significantly better than a prior art gasifier by Lurgi.
An efficient gasifier is identified as one having the characteristics of:
(a) using a low steam to dry, ash-free coal weight ratio of less than 1.0 or a low ratio of steam to syngas produced of less than 30 lbs. steam per mscf syngas; PA0 (b) producing a syngas with an H.sub.2 /CO ratio equal to or less than 1; and PA0 (c) a low temperature of the gasifier exit gas of less than 2000.degree. F.
Examples of gasifiers satisfying the above characteristics include slagging type gasifiers, such as the British Gas Corporation-Lurgi slagger or the Secord-Grate slagging gasifier or fluidized bed gasifiers, such as the U-Gas and Westinghouse gasifiers.
Any gasifier capable of producing a synthesis gas is applicable in the present invention. The significance of the above discussion lies in the efficiency which is gained by producing low H.sub.2 /CO ratio syngas. Such low ratio syngas when reacted in a Fischer-Tropsch system is highly exothermic and, as such, a slurry Fischer-Tropsch process is highly desirable to remove process heat as previously described.
Often, the catalyst employed for Fischer-Tropsch synthesis are complete free from activators and consist essentially of the metals of Group VIII of the Periodic Table as defined above, or their compounds only. It is thus desirable, and often required, to add suitable activating substances conventionally known and used for synthesis of the Fischer-Tropsch type. Copper, for example, is such an activating additive for iron catalyst. Cobalt or nickel may be activated by the addition of Th, Mg or Cu or their compounds. For a further increase in activity, alkali compounds are often added to the catalysts. For example, in U.S. Pat. No. 2,671,103, a slurry Fischer-Tropsch reaction system is operated to maintain a predetermined range of alkali content by adding to the catalyst material to be freshly introduced or adding together with such material a somewhat higher alkali content than is ordinarily contained in the catalyst material within the reactor at that time. Suitable alkali compounds are the oxides, hydro-oxides, carbonates, hydrocarbonates, phosphates, silicates and borates of sodium and potassium, furthermore their formates, acetates or the salts of higher organic acids, such as soaps.
The organic product formed in a slurry Fischer-Tropsch process contains olefins, paraffins and oxygenated hydrocarbons with carbon numbers from 1 to about 80. Only those compounds vaporized at the reactor conditions plus some entrained molecules will appear in the overhead effluent. The remainder, a high molecular weight wax, remains in the slurry oil. At conditions which yield only a few percent methane, 25% by weight or more of the product may be high molecular weight wax. At certain reaction conditions the viscosity of the wax increases during reaction, such that the slurry nears the point of gellation. A slurry medium is said to have gelled when it will not flow under gravity. If the slurry approaches gellation at reaction conditions, the reaction must be shut down.
During normal operation, the high molecular weight wax is periodically withdrawn to prevent build-up of such wax in the reactor and to prevent gellation of the slurry and reactor shutdown. When such draw-off of the wax is made, the wax contains entrained catalyst fines which need to be removed prior to upgrading the wax and which must be available for use in the process. In an article by R. Farley and D. J. Ray, Journal of the Institute of Petroleum, Vol. 50, No. 482, p. 31, February 1964, there is described the use of laboratory tests in order to find a method of wax withdrawal from a Fischer-Tropsch reactor which was capable of high withdrawal rates and which would still leave the catalyst available for use within the plant. Various magnetic, sintered-metal, and woven-wire cloth filters were shown to be unsuitable for the conditions of temperature, pressure or particle size utilized, and none of the filtering methods investigated gave adequate throughputs.
In "Magnetic Separation in Chemistry and Biochemistry", by Bernard L. Hirschbein et al, Chemtech, March 1982, pp. 172-178, the authors contend that use of magnetic separation can be increased in chemcial applications. Among suggested uses of magnetic separation is the removal of used catalyst-derived impurities from products such as Fischer-Tropsch catalysts. The article discloses that simple methods of generating very high magnetic field gradients have been developed. One such simple design comprises magnetic stainless steel wool pushed into a tube and the tube placed between the poles of magnet. Magnetic particles, such as in an aqueous suspension, would be retained by the steel wool and the water carrier would pass through unhindered. The high magnetic gradient is achieved by the intersection of the external magnetic field of the magnet and the local magnetic field produced by the magnetic filter element, e.g., steel wool, steel rods, filaments, etc.
An object of the present invention is to provide an improved process for catalytically hydrogenating carbon oxides in the presence of a finely divided catalyst in liquid suspension.
Another object of the invention is to provide an improved process for the removal of catalyst fines from the wax product which is recovered from a slurry Fischer-Tropsch reactor. A further object is to provide an improved process for the recovery and recycle of catalyst fines which are contained in the high molecular weight products removed from a slurry Fischer-Tropsch reactor. Still another object is to remove other undesirable compounds from the wax product which is removed from the slurry Fischer-Tropsch reactor so as to yield a pure wax product prior to wax upgrading. Other objects will become apparent from the following description of the invention.